There is another optional argument
which is the format string.
It indicates the color and line type
of the plot.
The letters and symbols of the
format string are from MATLAB
The default format string is ‘b-‘,
which is a solid blue line.
For example, to plot the previous
example with red circles, you would write
plt.plot([1,2,3,4], [1,4,9,16], "r1")